FWIW, I successfully used my unofficially supported Galaxy S9 in India. I did hit one snag, which is that I had bought the phone prior to going on the trip and hadn't installed the Project Fi app yet which meant I couldn't connect to any network after landing. Once I got on wifi and installed the app, I had 4G during my entire stay even in the most rural areas on overnight trains and no issues.
Though had I bothered to go through the whole passport photo/scan 24hr submission process for Airtel it would have been way cheaper.